Background Microwave plasma excitation sources are applied in various analytical techniques to excite an analytical sample being fed to a plasma zone. Due to a high plasma temperature, electrodes applied in capacitive excitation sources, which are also called antennas, may reach high surface temperature values related to thermal conduction and plasma radiation. When a certain critical density of power supplied to one electrode has been exceeded, depending on the above-mentioned factors, analytical lines related to the electrode material show up in the emission spectrum and, if the power density still increases, the electrode may even get damaged. The described processes restrain such significant parameters of a spectral source as electron density number and excitation temperatures. Intensive electrode cooling is necessary for a potential application of sufficiently high source parameters.
In typical microwave excitation sources in which the microwave resonator is unified with a 2.45 GHz microwave energy generator, the resonator is cooled with systems using a cooling gas or liquid which does not absorb a microwave radiation. A water aerosol may be also used as such a medium because radiation absorption by water particles sprayed in a gas is weak. If water is used, a cooling system must be located outside the microwave field force line concentration zone, e.g. in the microwave resonator walls or on the discharge tube circumference to avoid excessive microwave radiation absorption by water.
U.S. Pat. No. 5,568,015 patent specification describes a microwave plasma excitation source in which the microwave plasma generator resonator is made of a dielectric material and the cooling agent is essentially supplied via a spiral channel remaining in contact with the resonator wall and an external medium which does not absorb microwaves. In such a system, water can be used as a cooling agent due to the appropriate situation of a spiral channel in relation to the microwave field force lines. This solution is intended for a single-electrode plasma excitation source.
A method and system for plasma heating in a multi-phase electrode system is known from PL 385484 patent application. The method is that at least three electromagnetic waves are supplied by at least three identical elements of a plasma generator to the points which are symmetrically distributed on a circle around the generated plasma. As a result of a gas flowing through the circle center, plasma gets a toroidal shape through the center of which outside particles are fed. The system is equipped with at least one power generator and at least three identical electrodes distributed symmetrically on a circle in the center of which the plasma generator discharge tube is placed perpendicularly to the electrode system. The discharge tube incorporates a tube which feeds the outside particles to the plasma excitation area. The length of the electrodes used in such excitation sources is usually ¼ L or equal to an odd multiple of this length, where L is the length of a microwave generated by a microwave energy source and the electrode length is measured from its top to the power supply point. If the power supply point is connected to the electrode at a microwave maximum, the electrode length can be also ¾ L or its multiple.